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XML class for processing and building simple XML documents

By , 23 Sep 2003
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This article has been re-written with the help of 2 years of feedback, and the new source code has benefited from all of the fixes and developments during that time period. See release notes below.

Introduction

Often times you don't want to invest in learning a complex XML tool to implement a little bit of XML processing in your application. Its SO Easy! Just add Markup.cpp and Markup.h to your Visual C++ MFC project, #include "Markup.h", and begin using it. There are no other dependencies.

Features

  • Light: one small class that maintains one single document string with a simple array of indexes
  • Fast: the parser builds the index array in one quick pass
  • Simple: EDOM methods make it ridiculously easy to create or process XML strings
  • Independent: compiles into your program without requiring MSXML or any tokenizer
  • UNICODE: can be compiled for UNICODE for Windows CE and NT/XP platforms (define _UNICODE)
  • UTF-8: when not in UNICODE or MBCS builds, it works with UTF-8, ASCII, or Windows extended sets
  • MBCS: can be compiled for Windows double-byte character sets such as Chinese GB2312 (define _MBCS)

XML for Everyday Data

We often need to store and/or pass information in a file, or send a block of information from computer A to computer B. And the issue is always the same: How shall I format this data? Before XML, you might have considered "env" style e.g. PATH=C:\WIN95; "ini" style (grouped in sections); comma-delimited or otherwise delimited; or fixed character lengths. XML is now the established answer to that question except that programmers are sometimes discouraged by the size and complexity of XML solutions when all they need is something convenient to help parse and format angle brackets. For good minimalist reading on the syntax rules for XML tags, I recommend Beginning XML - Chapter 2: Well-Formed XML posted here on the Code Project.

XML is better because of its flexible and hierarchical nature, plus its wide acceptance. Although XML uses more characters than delimited formats, it compresses down well if needed. The flexibility of XML becomes apparent when you want to expand the types of information your document can contain without requiring every consumer of the information to rewrite processing logic. You can keep the old information identified and ordered the same way it was while adding new attributes and elements.

CMarkup Lite Methods

CMarkup is based on the "Encapsulated" Document Object Model (EDOM), the key to simple XML processing. Its a set of methods for XML processing with the same general purpose as DOM (Document Object Model). But while DOM has numerous types of objects, EDOM defines only one object, the XML document. EDOM harks back to the original attraction of XML which was its simplicity. To keep overhead low, CMarkup takes a very light non-conforming non-validating approach to XML, and it does not verify the XML is well-formed.

The CMarkup "Lite" in this article is the free version of the CMarkup product sold at firstobject.com. CMarkup Lite implements a subset of EDOM methods for creating and parsing XML document strings. The Lite methods also encompass some modification functionality such as setting an attribute or adding additional elements to an existing XML document, but not changing the data of, or removing, XML elements. See the EDOM specification to compare the full CMarkup with CMarkup Lite. The full CMarkup is available in Evaluation (Educational) and licensed Developer versions with many more methods, STL and MSXML versions, Base64, and additional documentation. But this Lite version here at Code Project is more than adequate for parsing and creating simple XML strings in MFC.

The CMarkup Lite methods are grouped into Creation and Navigation categories listed below.

CMarkup Lite Creation Methods

CString GetDoc() const { return m_csDoc; };
bool AddElem( LPCTSTR szName, LPCTSTR szData=NULL );
bool AddChildElem( LPCTSTR szName, LPCTSTR szData=NULL );
bool AddAttrib( LPCTSTR szAttrib, LPCTSTR szValue );
bool AddChildAttrib( LPCTSTR szAttrib, LPCTSTR szValue );
bool SetAttrib( LPCTSTR szAttrib, LPCTSTR szValue );
bool SetChildAttrib( LPCTSTR szAttrib, LPCTSTR szValue );

GetDoc is used to get the document string after adding elements and setting attributes. The AddAttrib and SetAttrib methods do the same thing as each other (as do AddChildAttrib and SetChildAttrib). They will change the attribute's value if it already exists, and add the attribute if it doesn't.

CMarkup Lite Navigation Methods

bool SetDoc( LPCTSTR szDoc );
bool IsWellFormed();
bool FindElem( LPCTSTR szName=NULL );
bool FindChildElem( LPCTSTR szName=NULL );
bool IntoElem();
bool OutOfElem();
void ResetChildPos();
void ResetMainPos();
void ResetPos();
CString GetTagName() const;
CString GetChildTagName() const;
CString GetData() const;
CString GetChildData() const;
CString GetAttrib( LPCTSTR szAttrib ) const;
CString GetChildAttrib( LPCTSTR szAttrib ) const;
CString GetError() const;

When you call SetDoc it parses the szDoc string and populates the CMarkup object. If it fails, it returns false, and you can call GetError for an error description. The IsWellFormed method returns true if the CMarkup object has at least a root element; it does not verify well-formedness.

Using CMarkup

The CMarkup class encapsulates the XML document text, structure, and current positions. It has methods both to add elements and to navigate and get element attributes and data. The locations in the document where operations are performed are governed by the current position and the current child position. This current positioning allows you to work with the XML document without instantiating additional objects that point into the document. At all times, the object maintains a string representing the text of the document which can be retrieved using GetDoc.

Check out the free firstobject XML editor which generates C++ source code for creating and navigating your own XML documents with CMarkup Lite.

Creating an XML Document

To create an XML document, instantiate a CMarkup object and call AddElem to create the root element. At this point, if you called AddElem("ORDER") your document would simply contain the empty ORDER element <ORDER/>. Then call AddChildElem to create elements under the root element (i.e. "inside" the root element, hierarchically speaking). The following example code creates an XML document and retrieves it into a CString:

CMarkup xml;
xml.AddElem( "ORDER" );
xml.AddChildElem( "ITEM" );
xml.IntoElem();
xml.AddChildElem( "SN", "132487A-J" );
xml.AddChildElem( "NAME", "crank casing" );
xml.AddChildElem( "QTY", "1" );
CString csXML = xml.GetDoc();

This code generates the following XML. The root is the ORDER element; notice that its start tag <ORDER> is at the beginning and end tag </ORDER> is at the bottom. When an element is under (i.e. inside or contained by) a parent element, the parent's start tag is before it and the parent's end tag is after it. The ORDER element contains one ITEM element. That ITEM element contains 3 child elements: SN, NAME, and QTY.

<ORDER>
<ITEM>
<SN>132487A-J</SN>
<NAME>crank casing</NAME>
<QTY>1</QTY>
</ITEM>
</ORDER>

As shown in the example, you can create elements under a child element by calling IntoElem to move your current main position to where the current child position is so you can begin adding under what was the child element. CMarkup maintains a current position in order to keep your source code shorter and simpler. This same position logic is used when navigating a document.

Navigating an XML Document

The XML string created in the above example can be parsed into a CMarkup object with the SetDoc method. You can also navigate it right inside the same CMarkup object where it was created; just call ResetPos if you want to reset the current position back to the beginning of the document.

In the following example, after populating the CMarkup object from the csDoc string, we loop through all ITEM elements under the ORDER element and get the serial number and quantity of each item:

CMarkup xml;
xml.SetDoc( csXML );
while ( xml.FindChildElem("ITEM") )
{
    xml.IntoElem();
    xml.FindChildElem( "SN" );
    CString csSN = xml.GetChildData();
    xml.FindChildElem( "QTY" );
    int nQty = atoi( xml.GetChildData() );
    xml.OutOfElem();
}

For each item we find, we call IntoElem before interrogating its child elements, and then OutOfElem afterwards. As you get accustomed to this type of navigation you will know to check in your loops to make sure there is a corresponding OutOfElem call for every IntoElem call.

Adding Elements and Attributes

The above example for creating a document only created one ITEM element. Here is an example that creates multiple items loaded from a previously populated data source, plus a SHIPMENT information element in which one of the elements has an attribute. This code also demonstrates that instead of calling AddChildElem, you can call IntoElem and AddElem. It means more calls, but some people find this more intuitive.

CMarkup xml;
xml.AddElem( "ORDER" );
xml.IntoElem(); // inside ORDER
for ( int nItem=0; nItem<aItems.GetSize(); ++nItem )
{
    xml.AddElem( "ITEM" );
    xml.IntoElem(); // inside ITEM
    xml.AddElem( "SN", aItems[nItem].csSN );
    xml.AddElem( "NAME", aItems[nItem].csName );
    xml.AddElem( "QTY", aItems[nItem].nQty );
    xml.OutOfElem(); // back out to ITEM level
}
xml.AddElem( "SHIPMENT" );
xml.IntoElem(); // inside SHIPMENT
xml.AddElem( "POC" );
xml.SetAttrib( "type", csPOCType );
xml.IntoElem(); // inside POC
xml.AddElem( "NAME", csPOCName );
xml.AddElem( "TEL", csPOCTel );

This code generates the following XML. The root ORDER element contains 2 ITEM elements and a SHIPMENT element. The ITEM elements both contain SN, NAME and QTY elements. The SHIPMENT element contains a POC element which has a type attribute, and NAME and TEL child elements.

<ORDER>
<ITEM>
<SN>132487A-J</SN>
<NAME>crank casing</NAME>
<QTY>1</QTY>
</ITEM>
<ITEM>
<SN>4238764-A</SN>
<NAME>bearing</NAME>
<QTY>15</QTY>
</ITEM>
<SHIPMENT>
<POC type="non-emergency">
<NAME>John Smith</NAME>
<TEL>555-1234</TEL>
</POC>
</SHIPMENT>
</ORDER>

Finding Elements

The FindElem and FindChildElem methods go to the next sibling element. If the optional tag name argument is specified, then they go to the next element with a matching tag name. The element that is found becomes the current element, and the next call to Find will go to the next sibling or matching sibling after that current position.

When you cannot assume the order of the elements, you must reset the position in between calling the Find method. Looking at the ITEM element in the above example, if someone else is creating the XML and you cannot assume the SN element is before the QTY element, then call ResetChildPos() before finding the QTY element.

To find the item with a particular serial number, you can loop through the ITEM elements and compare the SN element data to the serial number you are searching for. This example differs from the original navigation example by calling IntoElem to go into the ORDER element and use FindElem("ITEM") instead of FindChildElem("ITEM"); either way is fine. And notice that by specifying the "ITEM" element tag name in the Find method we ignore all other sibling elements such as the SHIPMENT element.

CMarkup xml;
xml.SetDoc( csXML );
xml.FindElem(); // ORDER element is root
xml.IntoElem(); // inside ORDER
while ( xml.FindElem("ITEM") )
{
    xml.FindChildElem( "SN" );
    if ( xml.GetChildData() == csFindSN )
        break; // found
}

Encodings

ASCII refers to the character codes under 128 that we have come to depend on, programming in English. Conveniently if you are only using ASCII, UTF-8 encoding is the same as your common ASCII set.

If you are using a character set not corresponding to one of the Unicode sets UTF-8, UTF-16 or UCS-2, you really should declare it in your XML declaration for the sake of interoperability and viewing it properly in Internet Explorer. Character sets like ISO-8859-1 (Western European) assign characters to the values in a byte between 128 and 255, so that every character still only uses one byte. Windows double-byte character sets such as GB2312, Shift_JIS and EUC-KR use one or two bytes per character. For these Windows charsets, put _MBCS in your preprocessor definitions and make sure your user's Operating System is set to the corresponding code page.

To prefix your XML document with an XML declaration such as <?xml version="1.0" encoding="ISO-8859-1"?>, pass it to SetDoc or the CMarkup constructor. Include a CRLF at the end as shown so that the root element goes on the next line.

xml.SetDoc( "<?xml version=\"1.0\" encoding=\"ISO-8859-1\"?>\r\n" );
xml.AddElem( "island", "Curaçao" );

Depth First Traversal

You can use the following code to loop through every element in your XML document. In the part of the code where you process the element, every element in the document (except the root element) will be encountered in depth first order. For illustrative purposes, it gets the tag name of the element. If you were searching for a particular element tag name you could break out of the loop at this point. "Depth first" means that it traverses all of an element's children before going to its sibling.

BOOL bFinished = FALSE;
xml.ResetPos();
if ( ! xml.FindChildElem() )
    bFinished = TRUE;
while ( ! bFinished )
{
    // Process element
    xml.IntoElem();
    CString csTag = xml.GetTagName();

    // Next element (depth first)
    BOOL bFound = xml.FindChildElem();
    while ( ! bFound && ! bFinished )
    {
        if ( xml.OutOfElem() )
            bFound = xml.FindChildElem();
        else
            bFinished = TRUE;
    }
}

Loading and Saving Files

CMarkup Lite does not have Load and Save methods. To load a file, look in the CMarkupDlg::OnButtonParse method which loads a file into a string. Once you have it in a string, you can put it into the CMarkup object using SetDoc. To save it to a file, call GetDoc to get the string and then implement your own code to write the string to your file. When you need to implement any of your own project specific I/O error handling, streaming, permissions/locking, and charset conversion, it is actually good software design to keep this outside of the CMarkup class allowing CMarkup to remain a generic class.

The Test Dialog

The Markup.exe test bed for CMarkup is a Visual Studio 6.0 MFC project (also compiles in VS .NET too). When the dialog starts, it performs diagnostics in the RunTest function to test CMarkup in the context of the particular build options that have been selected. You can step through the RunTest function to see a lot of examples of how to use CMarkup. Use the Open and Parse button in the dialog to test a file.

In the following illustration, the Build Version is shown as "CMarkup Lite 6.5 Debug Unicode." This means that it is the debug version built with _UNICODE defined. The RunTest completed successfully. A parse error was encountered in the order_e.xml file. It also shows the load and parse times, and file size.

The Test Dialog keeps track of the last file parsed and the dialog screen position for convenience. This is kept in the registry under HKEY_CURRENT_USER/ Software/ First Objective Software/ Markup/ Settings.

How CMarkup Works

The CMarkup strategy is to leave the data in the document string and maintain a hierarchical arrangement of indexes mapping out the document.

  • increase speed: parse in one pass and maintain hierarchy of indexes
  • reduce overhead: do not copy or break up the text of the document

CMarkup parses the 250k play.xml sample document in about 40 milliseconds (1/25th of a second) on a 500Mhz machine, holding it as a single string, and allocating about 200k for a map of the 6343 elements. From then on, navigation does not require any parsing. As a rule of thumb, the map of indexes takes up approximately the same amount of memory as the document, so the memory footprint of the CMarkup object should settle down around 2 times the size of the document. For each element in the document a struct of eight integers (32 bytes) is maintained.

int nStartL;
int nStartR;
int nEndL;
int nEndR;
int nReserved;
int iElemParent;
int iElemChild;
int iElemNext;

Look at the start and end tags in <QTY>1</QTY>. The struct contains the offsets of the left and right of both the start and end tags (i.e. all the < and > signs). The reserved integer is not currently used but could be used for a delete flag and/or level (i.e. depth) in the hierarchy to support indentation. The other three integers are indexes to the structs for the parent, child and next elements.

When the document is first parsed an array of these structs is built, and then as elements are modified and inserted in the XML, the structs are modified and added. Rather than allocating structs individually, they are allocated in an array using a "grow-by" mechanism to reduce the number of allocations to a handful. That is why integer array indexes rather than pointers are used for the links. Once an element is assigned an index in the array, that index does not change. So the index can be used as a way of referring to and locating an element

Release Notes

This release 6.5 of CMarkup Lite's public methods are backwards compatible with the previous release 6.1 posted here in August 2001 except for one rare usage of IntoElem. In 6.1, if you called IntoElem without a current child element, it would find the first child element. Now in 6.5 when there is no current child position, IntoElem puts the main position before the first child element so that a subsequent call to FindElem will not bypass the first element. So, the quick way to check this when upgrading is to scan all occurrences of IntoElem and make sure the previous CMarkup navigation call is FindChildElem before it. Or, if the child element was just created with AddChildElem then its okay because that sets the current child position too. For full details on this, see the IntoElem Changes in Release 6.3.

Other major changes since 6.1:

  • Fix: MBCS double-byte text x_TextToDoc *thanks knight_zhuge
  • Performance: parsing is roughly twice as fast
  • Debugging: see m_pMainDS and m_pChildDS class members while debugging to see string pointers showing current main and child positions
  • New Test Dialog interface with diagnostic results and load vs. parse times, and RunTest code for startup

License

CMarkup Lite is free for compiling into your commercial, personal and educational applications. Modify it as much as you like, but retain the copyright notice in the source code remarks. Redistribution of the modified or unmodified CMarkup Lite class source code is limited to your own development team and it cannot be made publicly available or distributable as part of any source code library or product, even if that offering is free. For source code products that derive from or utilize CMarkup Lite, please refer users to this article to obtain the source files for themselves. You are encouraged to discuss this source code and share enhancements here in the discussion board under this article. Enjoy!

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

About the Author

Ben Bryant

United States United States
Raised in Southern Ontario Canada. Bachelor of Science from the University of Toronto in Computer Science and Anthropology. Living near Washington D.C. in Virginia, USA.

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QuestionHow to delete the data from the XML file PinmemberMember 927486425-Jul-12 23:08 

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